1. Field of the Invention
The present invention relates to polycarbodiimides and a method for producing the same. More particularly, it relates to polycarbodiimides with superior hydrophilicity and crosslinking properties.
2 . Description of the Related Arts
Originally, industrial coatings were primarily solvent-borne systems, but the development of water-borne coatings has come to be of increasing interest for a number of reasons. The main reasons for the shift from solvent-borne coatings to aqueous alternatives are a decrease in the potential harm to the environment and lower toxicity. In the development of water-borne coatings, although the performance is often inferior to that of solvent-borne coatings, it is improved to a large extent by the addition of crosslinking agents such as water-dispersible polycarbodiimides.
Polycarbodiimides and their derivatives have been widely used as crosslinking agents or condensation agents in the preparation of carboxylic acids, sulfonic acids, esters, amidines, anhydrides, amides, and peptides. Previously, polycarbodiimides were prepared by reacting mercury or lead oxide with thioureas, and mercuric oxide and sodium hypochlorite were employed as catalysts later on; however, the large quantities of byproducts remained the greatest shortcoming. In recent years, polycarbodiimides were commonly obtained by heating diisocyanates at elevated temperatures by using phosphorous catalysts. Such a method has the advantages of less byproducts and a higher product yield while prolonging the reaction time. The reaction involves a combination of two isocyanate moieties to yield a carbodiimide group with evolution of carbon dioxide: EQU 2 R--NCO.fwdarw.RN.dbd.C.dbd.NR+CO.sub.2
Aliphatic and aromatic carbodiimides are liquid or solid at room temperature and it is often difficult to keep them stabilized during long-term storage since they tend to dimerize and/or trimerize at temperatures as low as room temperature. The rate of dimerization and/or trimerization will be affected by the nature of the terminal group attached to each carbodiimide group.
U.S. Pat. No. 5,081,173 discloses a surface-active polycarbodiimide containing a hydrophilic portion and a hydrophobic portion joined through the reaction of a carbodiimide group with a reactive functional group. U.S. Pat. No. 5,258,481 discloses a multi-functional water-dispersible crosslinking agent which consists of oligomeric compounds containing carbodiimide functions and other reactive functional groups to improve the efficiency of the crosslinking agent. U.S. Pat. No. 5,117,059 discloses mono-disperse, multi-functional carbodiimides which provide greater reaction efficiency at lower temperatures. U.S. Pat. No. 4,072,712 discloses a method for preparing carbodiimide-isocyanate adducts. U.S. Pat. No. 5,047,588 discloses a method for preparing surface-active polycarbodiimides of the formula: EQU R--X--R'
wherein X represents carbodiimide groups; R is a residue of a hydrophobic organic compound; and R' is a residue of an organic compound having a hydrophilic segment.
Generally, conventional polycarbodiimide crosslinkers contain a hydrophilic group at one end and a hydrophobic group at the opposite end. Such a structure, however, usually lacks hydrophilicity and powerful mechanical agitation is therefore required to disperse it in water. Usually, this requires the employment of special, high-shear mixing equipment, and results in the further drawback of high energy costs. Moreover, owing to the poor hydrophilicity, to guarantee a more stable dispersion, a longer hydrophobic chain is not possible, and therefore the total functional carbodiimide groups in a conventional polycarbodiimide molecular chain is restricted.